The invention generally relates to methods of making photovoltaic devices. More particularly, the invention relates to methods of making photovoltaic devices having an absorber layer.
Thin film solar cells or photovoltaic devices typically include a plurality of semiconductor layers disposed on a transparent substrate, wherein one layer serves as a window layer and a second layer serves as an absorber layer. The window layer allows the penetration of solar radiation to the absorber layer, where the optical energy is converted to usable electrical energy. Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-based photovoltaic cells are one such example of thin film solar cells.
Cadmium telluride (CdTe)-based photovoltaic devices typically demonstrate relatively low power conversion efficiencies, which may be attributed to a relatively low open circuit voltage (VOC) in relation to the band gap of the material which is due, in part, to the low effective carrier concentration and short minority carrier lifetime in CdTe. Effective carrier concentration of CdTe may be improved by doping with p-type dopants.
Further issues with improving the cell efficiency of CdTe solar cells include the high work function of CdTe and high back-contact resistance at the interface between CdTe and metal-based back contact layer. The back-contact resistance may be improved by increasing the carrier concentration at the back interface. For example, for a p-type CdTe material, increasing the carrier concentration amounts to increasing the p-type carriers in the CdTe material to form an “ohmic contact layer” on the backside of the CdTe layer, which is in contact with the back contact layer.
Typical methods employed to form the ohmic layers or for doping the absorber layer include incorporation of copper into back end of line processing of the absorber layer. However, photovoltaic devices including copper may lack desired levels of long-term stability.
Thus, there is a need for improved methods of making photovoltaic devices. Further, there is a need for improved photovoltaic device configurations having doped absorber layers and higher carrier densities at the back contact interface, resulting in higher efficiencies.